The present invention relates to a high-frequency current generating circuit for reading a signal recorded on an optical-recording medium which is a disk-type data storage medium (to be referred to as “optical disk” hereinafter) from/to which information is read and/or written by a laser beam, and relates to a controller for controlling the light intensity of a laser diode.
Recently, optical disk devices such as a CD-ROM (Compact Disk Read-Only Memory) device, a CD-R (Compact Disk Recordable) device, a CD-RW (Compact Disk Rewritable) device and DVD (Digital Versatile Disk) device, are widely used as storage devices provided externally of, for example, a personal computer which is an information processor for processing information. In addition, portable equipment such as a notebook computer handy to carry and mobile equipment which is easily movable, are popular. As a consequence, demand for increasing the storage capacitance of an optical disk device installed in these mobile equipment and for making the optical disk smaller in size, thinner and lighter, is rising.
Among constituent elements of an optical disk device, an optical pickup device for reading a signal recorded on an optical disk, includes many optical components and elements such as a laser diode emitting a laser beam by means of a semiconductor, an objective lens for converging the laser beam of the laser diode, a mirror for reflecting the laser beam, a hologram element for interfering with the laser beam and a light detector for detecting a laser beam. The capacitance and weight of these components and elements are large in an overall optical disk device. For that reason, to provide a smaller, thinner and lighter optical disk device largely depends on the optical pickup device. Due to this, various improvements have been hitherto made for optical pickup devices.
When a laser diode is employed in an optical pickup device, a problem of occurrence of so-called return-light noise arises. The occurrence of the return-light noise is a phenomenon that when a light beam emitted from the laser diode is applied onto an optical disk, a part of the reflected light returns from a optical disk surface to the laser diode to thereby generate noise. Since the laser diode is oscillated in a single mode having a single frequency, mode hopping that is frequency changes occurs when there is return-light. Then, the intensity of output light varies greatly, and return-light noise occurs as stated above.
To reduce return-light noise, the variation of the intensity of light may be prevented even if mode hopping occurs. To do so, there is known a method in which a laser of a laser diode is oscillated in multiple modes having two or more frequencies. According to this method, a high-frequency current having a frequency of 200 MHz or higher is superimposed on a control current for controlling the laser diode. The high-frequency current superimposed on the control current is a drive current for driving the laser diode. Consequently, the laser of the laser diode is oscillated in multiple modes by the drive current, and return-light noise is thereby reduced. In this case, the amplitude of the high-frequency current is not less than 20 to 30 mA.
When the above-stated method is carried out, it is preferable that an integrated circuit for generating a high-frequency current is arranged in the vicinity of the laser diode so as to reduce a driving loss and to make an optical pickup device smaller and thinner. This circuit, however, generates considerable heat. For example, the power supply voltage of a high-frequency current generating circuit is generally 5V, and power consumption thereof is not less than 300 mw. The laser diode has such properties that light emission characteristics deteriorate in a high temperature environment, and in particular, the laser diode may not emit light at a temperature of 80° C. or higher. It is, therefore, difficult to arrange the high-frequency current generating circuit in the vicinity of the laser diode. Nevertheless, a circuit for effectively reducing the power consumption of a high-frequency generation circuit is realized as disclosed by, for example, Japanese Patent Unexamined Application Publication (KOKAI) No. 10-247329. The circuit disclosed therein allows a high-frequency current generating circuit to be arranged in the vicinity of a laser diode. Additionally, according to Elantec Homepage (http://www.elantec.com), two output current loops circuit for reducing power consumption is shown on “EL6200C Preliminary Mar. 19, 1999”, page 6, FIG. 2.
However, to further accelerate data transmission rate at the time of reading data, the frequency of the high-frequency current is required to be as high as 300 to 500 MHz.
On the other hand, laser diodes can be produced at lower cost and operated at higher temperature, so that a junction capacitance thereof tends to be higher. When the frequency of such a superimposed high-frequency current increases and so does the junction capacitance of the laser diode increase, driving loss rises. If so, the amplitude of the superimposed high-frequency current needs to be increased further. As a result, the power consumption of the high-frequency current generating circuit increases, thereby making it quite difficult to arrange a high-frequency current generating circuit in the vicinity of the laser diode.
Meanwhile, a DVD drive or CD drive is provided with an optical system including a red laser diode (wavelength=650 nm) for reading data on the DVD and an optical system including a near-infrared laser diode (wavelength=780 nm) for reading data on the CD-R/RW. In an optical disk device provided with these two laser diodes emitting different wavelength light, two independent high-frequency current generating circuits corresponding to the respective laser diodes are prepared. This possibly leads to an increase in the size of an integrated circuit including a high-frequency current generating circuit for generating a high-frequency current and to hike cost up. At present, the development of a laser diode which emits a laser beam due to two wavelengths being switched is undergone. Even so, the same disadvantages occur if two independent high-frequency current generating circuits corresponding to the respective wavelength are provided.
Generally, an auto-power control (APC) is employed in the drive system of the laser diode. Specifically, a part of the light emitted from the laser diode is received by a light detector for monitoring the intensity of light and the output current is converted into a voltage by a current-to-voltage conversion resistor. Thereafter, the converted voltage is supplied to an APC circuit as a control signal for controlling the light intensity. The APC circuit generates a control current for controlling the intensity of light emitted from the laser diode. A high-frequency current stated above is superimposed on this control circuit. The laser diode and the light intensity monitored light detector are packaged in the same package.
When this APC is applied to the optical disk device provided with the two laser diodes having a different wavelength stated above, the following disadvantages occur. The first disadvantage is caused by providing light detectors for monitoring the intensity of light, these detectors being individually provided to correspond to the respective laser diodes. If so, the number of pins of the package for packaging the laser diode and the light detector of monitoring light intensity, disadvantageously increases. Another disadvantage is that if a monolithic structure in which two laser diodes are formed in one chip is employed, this structure requires an optical device, such as a dichroic prism, having wavelength selectiveness and separating an optical path so as to separate light of the respective wavelength from each other and to allow the separated light to be incident on the respective light-monitoring detectors. As a result, the size of the package becomes large. These disadvantages are serious obstructions to realizing a smaller, thinner and lighter optical pickup device.
It is considered that these disadvantages can be overcome by employing a light detector for monitoring light intensity, this detector being common to two laser diodes. The detection sensitivity of a light detector is normally dependent on wavelength of light, and an output current of the light detector varies according to the wavelength of an incident light with a fixed intensity of light. Accordingly, in one light-monitoring detector, it is difficult to accurately control the intensity of light of each laser diode.
As described above, according to the conventional high-frequency current superposition technique, if the frequency of the high-frequency current superimposed on the control current of the laser diode increases due to high rate of transmitting data, the junction capacitance of the laser diode increases and thereby driving loss increases, then it is required to increase the amplitude of the superimposed high-frequency current. Consequently, the power consumption of the high-frequency current generating circuit increases to thereby generate more heat. It is, therefore, difficult to provide a smaller, thinner optical pickup device by providing a high-frequency current generating circuit in the vicinity of a laser diode.